1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to a combination separator provided to electrically insulate the anode from the cathode in an electrochemical cell. The combination separator comprises at least one layer of a flexible, non-woven polyolefinic cloth superposed with at least one layer of a flexible, microporous polyolefinic film. A preferred polymeric material for both the non-woven cloth and the film is polypropylene. A preferred superposed combination separator according to the present invention has a surfactant such as a non-ionic surfactant coated thereon, and more preferably coated on the film. An electrochemical cell includes an alkali metal primary or secondary cell having the present combination separator intermediate the electrodes to provide physical separation therebetween.
2. Prior Art
Conventionally, separators have fallen into two general categories--those made of microporous films and those in the form of a cloth made from various materials including polymeric fibers. Whether a separator is comprised of a microporous film or a cloth material, a fundamental requirement is that the material of construction be resistant to degradation in the cell environment, have sufficient thickness to maintain interelectrode separation without interfering with cell discharge performance, and exhibit sufficient surface energy such that electrolyte wettability and absorption are augmented. The separator material must also have a relatively high electrical resistivity in order to prohibit the establishment of short circuit currents flowing directly between the electrodes through the separator.
An example of a film separator is shown in U.S. Pat. No. 4,629,666 to Schlaikjer, which discloses partially halogenated microporous polymeric films for use as separators in electrochemical cells containing alkali metals, such as lithium, and inorganic electrolytes. Similarly, a microporous film separator comprising polytetrafluoroethylene (PTFE) is disclosed in U.S. Pat. No. 3,661,645 to Strier et al. The benefit is that microporous films can be made very thin which contributes to volumetric efficiency in that the separator does not detract appreciably from the volume of cathode and anode active materials and therefore the energy density. The problem is that reduction in separator thickness is accompanied by a reduction in material strength as microporous films, including those made of polytetrafluoroethylene, can be weak and susceptible to tearing. As previously discussed, tensile properties are important in selecting a separator, and it is not uncommon for film separators to rupture during the manufacturing process, which can lead to contact between the electrode materials and result in an internal short circuit condition.
The separator also must have sufficient porosity such that electrode separation is maintained while allowing ionic transfer within the electrolyte to occur unimpeded during intended cell discharge. Cloth separators are relatively porous structures. An exemplary cloth separator is shown in U.S. Pat. No. 5,002,843 to Cieslak et al., which discloses a lithium/thionyl chloride electrochemical cell system having a separator made of aramid fibers provided in a non-woven mat form. Although non-woven mats are highly porous and, therefore, not a detriment to ionic transfer within the depolarizer/catholyte, their inherent porosity may allow small particles of electrode material to migrate through the separator. As is the case with microporous films, the use of a non-woven cloth as a separator in an electrochemical cell can result in direct physical contact between the electrodes, which would give rise to an internal short circuit condition.
The above requirements are balanced by the need that the separator have sufficiently strong tensile properties to facilitate cell fabrication and to further withstand internal cell stresses due to changes in electrode volume during discharge, and during re-charging cycles in secondary electrochemical cells. Polytetrafluoroethylene has high tensile strength and is, therefore, desirable for use as a separator material in some cell chemistries, especially when provided in a cloth form. To complicate matters, however, it is known that alkali metals such as lithium are reactive in contact with fluorinated carbon (CF.sub.x) electrode active materials and polytetrafluoroethylene separators. Contact between lithium metal electrodes and polytetrafluoroethylene separators can generate sparks and possibly sufficient heat to cause fire. Furthermore, while fluorinated carbons (CF.sub.x) are useful as cathode active materials, and especially in cells intended to be discharged under a light load for extended periods of time such as for routine monitoring of cardiac functions by an implantable cardiac defibrillator and the like, it is imperative that physical separation is maintained between lithium and the cathode material without the provision of polytetrafluoroethylene as a separator material due to the potential for excessive heat generation when fluorinated carbon active materials are contacted with alkali metals.
The separator combination of the present invention has excellent tensile properties while allowing ionic transfer within electrolyte to occur, but it does not include polytetrafluoroethylene. In comparison to separators made of that material, the use of one non-woven polyolefinic cloth superposed with a microporous polyolefinic film according to the present invention provides a separator combination for alkali metal cells having all of the desirable above-described attributes of a separator structure including the prevention of internal short circuit conditions without interfering with discharge performance while also allowing for the fabrication of thinner and/or smaller cell constructions.
It should be pointed out that an alkali metal electrochemical cell that does not use polytetrafluoroethylene as a separator material is described in U.S. Pat. No. 4,552,821 to Gibbard et al. However, this patent describes a separator comprising at least two layers of polypropylene film used in conjunction with a non-woven fabric wicking layer. The double layers of polypropylene film are intended to minimize dendritic short circuit conditions during recharging of the disclosed nickel-zinc secondary cell. In practice, the double layers of polypropylene separator film provide a degree of redundancy that allows large pores or holes, due to imperfections produced during manufacture or subsequently, in each film to be non-aligned to minimize short circuit conditions caused by dendrite growth during cell recharging.
Hoechst Celanese Corporation, Charlotte, N.C., sells a combination separator under the designation CELGARD.RTM. 5550 comprising a layer of non-woven polypropylene laminated with a layer of polypropylene film having a blend of non-ionic and cationic surfactants coated thereon. However, it is believed that the lamination process reduces this combination separator's ability to support ion transfer therethrough.
As previously discussed, the present alkali metal electrochemical cell can comprise either a primary cell or a secondary, rechargeable cell. A most preferred form of the present electrochemical cell includes a Li/CF.sub.x couple. In that case, it is imperative that physical separation is maintained between the respective electrode active materials to not only prevent the formation of short circuit conditions but also to preclude internal heat generation. Such an eventuality could be catastrophic.
In the present invention, redundant film layers are not desired. The present superposed non-woven cloth and microporous film provide improved strength characteristics in comparison to one or the other layer used alone and the present separator structure also prevents migration of fluorinated carbon cathode materials into short circuit contact with the anode material while still providing for cells of reduced thickness and smaller size.